This invention relates to the digital sampling of electrical waveforms. More particularly, the invention relates to providing, in a digital sampling oscilloscope, an effectively increased and controllably shaped pass-band through software control of existing hardware.
It is necessary in signal measuring instruments generally to provide a sufficient bandwidth in the instrument to respond to the highest frequencies of a signal to be measured within acceptable limits of distortion. As is well known, measuring instruments generally lose this capability at high enough frequencies, their response becoming substantially attenuated and phase shifted past a 3 db, break or cut-off frequency.
A standard approach to increasing the bandwidth of a signal measuring instrument is to employ faster circuitry to increase the physical sampling rate. However, faster circuitry has the disadvantage of higher cost; especially so where there are multiple channels in each of which it is desired to provide higher speed.
An alternative approach has been to provide for what is known in the art as equivalent time sampling ("ET" and "ET mode"). In ET mode, advantage is taken of the periodic nature of the waveform to be sampled wherein multiple, sequential acquisitions of samples of the waveform are acquired and employed to increasingly build up a higher resolution record of the period of the waveform. The samples in one acquisition are generally acquired asynchronously with the samples of other acquisitions so they do not overlap.
The number of acquisitions employed in ET mode can be seen to be the factor by which time is effectively expanded ("time expansion factor") or the factor by which the physical sampling rate is effectively multiplied. That is, a greater physical time is provided during which samples of the waveform are acquired so that, for a given period of the waveform, more samples are acquired. However, the greater physical time is provided by sampling over repeated periods of the waveform. Accordingly, it can be appreciated that ET sampling cannot generally be employed in the sampling of non-periodic waveforms.
Digital sampling oscilloscopes often provide for an ET mode. An acquisition memory register is typically arranged in a circular queue. Digitizing circuits write to the memory register at a substantially constant rate with samples of a waveform. When a triggering event triggers the oscilloscope, a predetermined number of samples are written into the memory register, whereafter sampling is ceased. The predetermined number of samples corresponding to the triggering event forms an "acquisition."
Trigger location information employed to locate the trigger and, hence, the start of the desired data in the acquisition memory is stored in a separate memory register. The contents of the acquisition memory register corresponding to a particular acquisition may then be located and moved into an ET record for ultimate display to a user of the oscilloscope.
Typically, software is responsible to locate, move and otherwise process an acquisition for display. Some of the software instructions (an "instruction set") are executed in proportion to the number of samples acquired, while some of the instructions are executed in proportion to the number of acquisitions, regardless of the number of samples in the acquisition. The latter instructions are, therefore, overhead with respect to the samples. Typically, overhead is a very large percentage of the total instruction set.
Moreover, in standard operation of the oscilloscope, the aforementioned instructions are invoked for one acquisition, one acquisition being an essentially complete result for display. However, in ET mode, where multiple acquisitions are required to provide the desired resulting display, the instructions must be invoked multiple times, i.e., for each acquisition. Therefore, it can be appreciated that ET mode not only employs a lower physical sampling rate, resulting in fewer samples per acquisition, but sacrifices significant speed as well by multiplying the overhead as a result of employing multiple acquisitions.
Digital sampling oscilloscopes also may provide for a "fast frame" mode. The acquisition memory register is partitioned into a number of frames or segments. Typically, the samples for a first acquisition are stored in one of the segments and the samples of next acquisitions are stored in following segments before invoking the aforedescribed overhead. With the overhead now amortized over a number of acquisitions, the effective sampling rate of the oscilloscope may be greatly increased. The acquisitions are concatenated for ultimate presentation to the user on the display. While this methodology provides for an increased sampling rate by decreasing the effective overhead for a given acquisition, the effective sampling rate remains limited by the speed of sampling and digitizing hardware.
Digital sampling oscilloscopes are sometimes employed to provide a so-called "eye diagram" for mask testing of a digital bit stream. Here, the signal being measured is generally non-periodic. The "eye" pattern, inter alia, displays the shape of the edges of the digital pulses representing bits of data. Consequently, acceptable resolution of eye diagram patterns requires a sampling frequency of at least twice the frequency of the Fourier components forming the edges of the digital pulses. Accordingly, it is especially advantageous in an eye diagram pattern to employ a digital sampling oscilloscope with a relatively capacious bandwidth compared to the bit rate.
Such eye pattern diagrams may be used for analysis of optical waveforms such as Synchronous Optical Networks and Fibre Channel optical waveforms ("SONET/Fibre Channel"), particularly mask testing. In such mask testing, standards are typically employed which specify the frequency response of a reference receiver (hereinafter "reference channel"), such as a digital sampling oscilloscope employed for constructing an eye pattern diagram. For example, the ITU-TS G 957 standard specifies a reference receiver having a 4th order Bessel-Thompson ("BT") frequency response whose 3 db bandwidth is selected to be 0.75 times the bit rate.
The reference channel generally includes a digital sampling oscilloscope, a probe connected to electrical inputs of the digital sampling oscilloscope, and a filter for equalizing the reference channel to substantially match the desired BT frequency response. Typically, the digital sampling oscilloscope employs a bandwidth that comfortably exceeds the desired frequency response. An example of such a digital sampling oscilloscope employed for optical mask testing is the HP 71501, manufactured by Hewlett Packard Company of Palo Alto, Calif. Typically as well, the probe is provided a bandwidth which exceeds the bandwidth of the desired BT response. Then, an expensive but standardized, outboard hardware filter is typically employed to shape the pass-band of the reference channel to fall off with the BT response. On the other hand, if the cut-off frequency of either the probe or the scope, or both, is in the region of the desired BT response, the hardware filter must, in addition, be tailored to the particular frequency response characteristics of the reference channel.
It is often undesirable to incur the cost of employing a reference channel having a sufficiently high frequency response to permit use of the standardized filter. This is particularly so where the bit rate is as high as 622 Mb/s, as for some SONET/Fibre Channel signals. And while a customized filter can generally be employed to obtain the desired frequency response in the reference channel, the disadvantages of requiring an expensive, customized filter for each combination of probe and scope are readily apparent.
Accordingly, there is a need for a method and apparatus for digital sampling of electrical waveforms that provides, at relatively low cost in a digital sampling oscilloscope, increased effective bandwidth and, for some applications, that combines increased effective bandwidth with improved control over the shape of the pass-band of the digital sampling oscilloscope.